Proceedings V World Avocado Congress (Actas V Congreso Mundial del Aguacate) 2003. pp. 335-342.
ENVIRONMENTAL REGULATION OF PHOTOSYNTHESIS IN
AVOCADO TREES – A MINI-REVIEW
B. Schaffer1 and A.W. Whiley2
1Tropical
Research and Education Center, University of Florida, 18905 SW 280 Street,
Homestead, Florida, 33031
2Sunshine
Horticultural Services Pty Ltd, 287 Dulong Road, Nambour 4560, Australia
SUMMARY
An important aspect of orchard management is to manipulate and train trees to optimize photosynthesis within the canopy. Knowledge of the impact of environmental factors, such as light, temperature, humidity, flooding, salinity and elevated atmospheric CO2 concentrations on photosynthesis of avocado trees provides information that can be applied to canopy management and plant
selection for specific environments. This mini-review summarizes the current knowledge of the
impact of environmental factors on avocado photosynthesis and its implications for crop management plant selection.
Key Words: Avocado, Persea americana, photosynthesis, environment.
INTRODUCTION
In avocado (Persea americana Mill.) trees, increasing carbohydrate partitioning to flowers and fruit
provides a challenge for orchard management, as the tree has a natural vegetative bias resulting
in a greater allocation of photoassimilates to shoot growth than to reproductive organs (Whiley et
al., 1988b; Wolstenholme, 1990; Schaffer and Whiley, 2002). This vegetative bias, coupled with
the relatively short leaf longevity (for a subtropical fruit tree species) results in rapid production of
short-lived leaves and increased shading within the canopy that reduces the number of well-lit terminal shoots capable of flowering (Wolstenholme and Whiley, 1999). Leaves of ‘Booth-8’ avocado
trees exhibit a net carbon lost (determined by summing photosynthesis and respiration) during the
first 21 days of development when they reached 72% of their full expansion (Schaffer et al., 1991).
Similarly, there was a net carbon gain for ‘Hass’ avocado leaves only after they reached 80% of full
expansion (Whiley, 1990). Thus, the rapid turnover of leaves results in photosynthetically produc-
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tive leaves being shading by younger leaves, which are sinks rather than sources for photoassimilates. This makes it difficult to train avocado canopies for optimum light interception and carbon
assimilation. Thus, a key to improving productivity of avocado is the development of management
strategies aimed at increasing the photosynthetic potential and realization by increasing light penetration within the canopy.
A key factor in developing efficient management strategies for avocado orchards is to maximize
photosynthetic efficiency within canopies. To do this an understanding of impact of environmental
factors on the regulation of photosynthesis is very helpful. The objective of this mini-review is to
briefly summarize the current knowledge of the impact of environmental factors on photosynthesis
of avocado.
Responses to Light
A key factor to avocado canopy management is to select smaller trees with better overall light
interception so that a high percentage of leaves within the canopy are above the light saturation
point for photosynthesis (Whiley and Schaffer, 1994; Wolstenholme and Whiley, 1999; Whiley,
2002). To achieve this goal an understanding of irradiance effects on photosynthesis is essential.
In an orchard the light saturation point for photosynthesis of mature ‘Hass’ avocado trees was
determined at a photosynthetic photo flux (PPF) of 1110 µmol quanta m-2 s-1
(Whiley, 1994), much higher than the PPF of 400-500 µmol quanta m-2 s-1 reported for containergrown ‘Fuerte’ trees (Scholefield et al., 1980). The light saturation of potted ‘Edranol’ trees was
determined at a PPF of 660 µmol quanta m-2 s-1 (Bower, 1978). The higher light saturation point
observed for potted ‘Edranol’ trees compared to the ‘Fuerte’ was presumably a result of ‘Edranol’
photosynthesis being measured in the whole canopy rather than on a single leaf basis as was done
with ‘Fuerte’. Thus mutual shading of ‘Edranol’ leaves undoubtedly resulted in a higher light intensity necessary to saturate all of the leaves in the canopy (Schaffer and Whiley, 2002). The much
greater light saturation point observed for field-grown trees may have been a result of root restriction in container-grown trees, which limited net photosynthesis to about 7 µmol CO2 m-2 s-1 compared to 23 µmol CO2 m-2 s-1 for trees in an orchard (Schaffer et al., 1999; Whiley and Schaffer,
1994; Schaffer and Whiley, 2002). Low net photosynthetic rates [7-10 µmol CO2 m-2 s-1 (Schaffer
et al., 1987; 1991)]of orchard trees in Florida compared to orchard trees elsewhere was attributed
root restriction in extremely hard Florida soils, which mimicked root restriction of container-grown
trees (Schaffer et al., 1994; Schaffer et al., 1999).
Avocado presumably evolved as a small gap-colonizing, understorey forest species (Whiley and
Schaffer, 1994; Wolstenholme and Whiley, 1999; Wolstenholme, 2002). Several physiological
attributes, such as a low light compensation (the PPF level at which net photosynthesis equals 0)
and the rapid “turn-over” of relatively short-lived leaves reflect avocado’s putative center of origin
(Whiley and Schaffer, 1994; Wolstenholme and Whiley, 1999; Wolstenholme, 2002). The light compensation point of approximately 10 mmol quanta m-2 s-1 for sun-lit leaves in a ‘Hass’ orchard
(Whiley, 1994; Whiley and Schaffer, 1994) would provide an adaptive advantage for harvesting light
for photosynthesis in a low-light understorey environment. Additionally, in its native habitat, the vegetative bias due to frequent replacement of short-lived leaves allowed a competitive advantage for
maximizing photosynthesis by frequent replacement of excessively shaded leaves. However, in an
orchard situation, this vegetative bias results in increased shading of the canopy (Wolstenholme
and Whiley, 1999).
Despite a considerable amount of research on management practices for avocado, compared to
temperate fruit crops there has been relatively little improvement in tree size control either through
breeding or canopy management (Whiley and Schaffer, 1994; Schaffer and Whiley, 2002). Also,
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light interception within avocado canopies over time has not been sufficiently quantified (Schaffer
and Whiley, 2002). Tree spacing, tree thinning, and mechanical or hand pruning techniques must
maximize light interception and utilization within avocado canopies. Often a combination of these
techniques must be employed to reduce excessive shading, either by over-crowding of trees or
excessive vegetative growth within canopies. A study of the dynamics of light interception in a
growing orchard and its impact on yield, would contribute significantly to more informed orchard
management (Whiley and Schaffer, 1994).
Responses to Temperature
There is a difference in temperature responses among the 3 ecological races (Mexican,
Guatemalan and Lowland) of avocado (Krezdorn, 1970; Whiley and Schaffer, 1994). Studies and
observations on growth and anatomical damage indicated that Mexican race cultivars are the most
cold-tolerant, whereas Guatemalan race cultivars are intermediate in cold tolerance and Lowland
(or West Indian) race cultivars are the least cold-tolerant (Schaffer and Whiley, 2002). Although
comparisons of photosynthetic responses to temperature among races have not been reported, it
stands to reason that the photosynthetic responses would most likely parallel anatomical damage
and growth responses to temperature.
Photosynthetic rates of avocado may be significantly affected by slight fluctuations in temperature.
For the ‘Edranol’, a Guatemalan hybrid cultivar (Newett et al., 2002), in containers, the optimum
temperature range for photosynthesis was 20-24°C (Bower et al., 1978). Within ± 5°C of this temperature range, net photosynthesis declined by about 20%. For container-grown ‘Fuerte’ trees (a
Mexican x Guatemalan hybrid; Newett et al., 2002), maximum net photosynthetic rates were
observed at temperatures of 28-31°C and that rate decline by about 33% at temperatures below
15°C or above 40oC (Scholefield et al., 1980). In an orchard in Queensland, Australia the maximum photosynthetic rate of ‘Hass’, a predominantly Guatemalan race cultivar with some Mexican
genes (Newett et al., 2002), decreased from 19.0 µmol CO2 m-2 s-1 during autumn, when minimum
daily temperatures were greater than 14°C, to 10.9 µmol CO2 m-2 s-1 in winter when minimum temperatures were less than 10°C (Whiley et al., 1999). It was also observed that that temperatures
lower than 10°C during winter significantly reduced apparent quantum yield of leaves of field-grown
‘Hass’ avocado trees from 0.055 µmol CO2 µmol-1 quanta to 0.034 µmol CO2 µmol-1 quanta (Whiley,
1994).
Chlorophyll fluorescence can be used to measure plant stresses caused by temperature extremes.
Photoinhibitory damage to Photosystem 2 (PS II) can be quantified by measuring a decrease in the
variable to maximum chlorophyll fluorescence (FV/Fm) ratio (Björkman, 1987, Demmig and Björkman, 1987). Whiley (1994) reported that the Fv/Fm ratios of avocado were 0.79-0.81 when minimum temperatures in an orchard were above 12.9°C. However, when minimum temperatures
dropped below 10°C, the mean Fv/Fm ratio was 0.41, indicating cold-induced damage to PS II.
In addition to measuring cold-induced photoinhibitory damage in avocado, chlorophyll fluorescence
has been used to indicate the presence of a heat acclimation mechanism in avocado leaves that
prevents photo-oxidative damage to PS II over a moderate increase in temperature from 21-35°C
(Havaux and Lannoye, 1987).
Responses to Humidity
The leaf water status of fruit crops is strongly tied to the diurnal fluctuations in the evaporative
demand of the environment. Thus, leaf water status varies much more in perennial fruit crops than
in annual crops, and leaf water stresses may occur under high evaporative demand even if soil water
content is adequate (Flore and Lakso, 1989). The effect of humidity on plant photosynthesis is often
a result of the effect of vapor pressure deficit. (VPD) on stomatal conductance although there can be
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non-stomatal photosynthetic responses to changing VPD (Schultze, 1986). Reports of the effects of
humidity on avocado have linked decreased photosynthetic responses to decreased stomatal conductance as a result of increasing VPDs (Sterne et al., 1977; Bower et al., 1978; Scholefield et al.,
1980). For leaves of ‘Fuerte’ avocado, stomatal conductance was high shortly after sunrise when
VPD was low but decreased significantly during the middle part of the day when VPD was much higher (Whiley et al., 1988a). However, net photosynthesis was not determined in that study. With ‘Edranol’ trees in containers, however, a 50% reduction in stomatal conductance due to increased VPD
resulted in a concomitant 50% decrease in net photosynthesis (Bower et al., 1978).
Responses to Drought
In recent studies, Neuhaus (2003) determined that reduced photosynthesis in avocado as a result
of soil water deficit stress is primarily due to reductions in stomatal conductance. Additionally, he
determined that stomatal conductance was a more reliable early indicator of water stress in avocado than measurements of leaf water content, leaf water potential, or growth variables.
In avocado, stomatal conductance begins to decline when leaf water potential is ª 0.4 MPa, and
continues to decline until stomatal closure occurs at leaf water potentials of 1.0 - 1.2 MPa (Sterne
et al., 1977; Bower, 1978; Scholefield et al., 1980; Whiley et al., 1988a). This decline in stomatal conductance is accompanied by a concomitant decline in net photosynthesis (Bower, 1978;
Ramadasan, 1980). When water stress is alleviated, leaf water potential has been observed to
recover more slowly than net photosynthesis (Ramadasan, 1980) and stomatal conductance
(Sterne et al., 1977; Bower et al., 1978; Ramadasan, 1980).
Neuhaus (2003) observed younger leaves of water-stressed plants had less control over water loss
and a lower rate of net photosynthesis than older leaves resulting in lower water use efficiency for
stressed young leaves. Based on that, Neuhaus (2003) concluded that avocado leaves are more
sensitive to water stress during “flushing” when there are a high percentage of young leaves.
Responses to Flooding
Avocado is considered a flood-sensitive species with physiological responses occurring shortly
after soils become waterlogged (Schaffer et al., 1992). In flooded soils, the decline in net photosynthesis of avocado is generally accompanied by decreases in stomatal conductance and intercellular partial pressures of CO2 in the leaves (Ploetz and Schaffer, 1989; Schaffer and Ploetz,
1989). However, the temporal separation between these physiological events has not been
defined, which would be useful for determining if flood-induced reductions in photosynthesis in avocado are due to stomatal or non-stomatal factors (Schaffer et al., 1992).
In Krome very gravely loam soil, flooding avocado trees with root systems that were 20% damaged
(necrosis) from previous Phytophthora root rot, resulted in almost a complete inhibition of photosynthesis. However, in this same soil non-flooded trees were able to sustain up to 50% root damage from Phytophthora without a decrease in net photosynthesis and up to 90% of the root system damaged with only 65% decrease in photosynthesis (Schaffer and Ploetz, 1989). Thus,
although avocado is considered a flood-sensitive species, the negative impact of flooding is greatly exacerbated by the presence of Phytophthora root (Ploetz and Schaffer, 1987, 1988, 1989;
Schaffer and Ploetz, 1987; Schaffer et al., 1992). Therefore, in poorly drained soils, floodinginduced inhibition of photosynthesis of avocado may be reduced by properly managing orchards
to control Phytophthora root rot.
Responses to Atmospheric CO2 Concentration
There have been very few reports on the effects of short- or long-term atmospheric CO2 enrichment on tropical fruit crops, including avocado. Short-term (5 min) exposure of ‘Hass’ avocado
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leaves to increasing atmospheric CO2 concentrations resulted in net photosynthesis increasing
asymptotically as the ambient CO2 concentration increased until maximum photosynthesis
occurred at atmospheric CO2 to concentrations of 1350-1470 µmol CO2 µmol-1. There are no
reports in the literature on the effects of long-term exposure to increased atmospheric CO2 on photosynthesis of avocado. However, growing ‘Hass’ avocado trees for 6 months in a CO2-enriched
environment (atmospheric CO2 concentration = 600 µmol mol-1) resulted in significantly more dry
matter accumulation compared to plants that were grown in a near ambient atmospheric CO2 concentration (350 µmol mol-1). Although photosynthesis was not measured in that study, it is safe to
assume that increased growth in the enhanced CO2 environment resulted from increased net photosynthetic rates due to constant exposure to elevated CO2 levels.
Photosynthetic efficiency was higher, although the actual photosynthetic rate was lower in callusderived, avocado shoot cultures and plantlets grown in ambient atmospheric CO2 than for cultures
and plantlets grown in an enhanced CO2 environment (Witjaksono et al., 1999). Thus, increasing
atmospheric CO2 concentration during plantlet and shoot development resulted in increased
growth. Therefore increasing atmospheric CO2 concentration during development is a useful technique proliferating enhanced growth of cultured avocado plants
Responses to Salinity
Avocado is considered a salt-sensitive species (Kadman, 1963, 1964; Downton, 1978) and there
have been some efforts to select rootstocks for salinity tolerance (Haas, 1950; Embleton et al.,
1961; Oster and Arpaia, 1992). Mickelbart and Arpaia (2002) observed that increasing salinity in
the root zone from 1.5 to 6.0 dS.m-1 resulted in up to a 23% decrease in net photosynthesis of
one-year-old ‘Hass’ avocado trees grafted on ‘Thomas’, ‘Duke 7’ (Mexican race cultivars) or ‘Toro
Canyon’ (a Mexican x Lowland race hybrid) rootstocks. There were no consistent differences
among cultivars with respect to the impact of salinity on net photosynthesis. Differences in sensitivity to salinity among cultivars were reflected in different growth reductions and leaf necroses
among cultivars in response to elevated salt concentrations in the rhizosphere rather than by photosynthetic responses.
CONCLUSIONS
A key factor in developing efficient management strategies for avocado orchards is to maximize
photosynthetic efficiency within canopies. Recent orchard studies indicate that under non-stress
conditions, the net photosynthetic rate of avocado is considerably higher that what earlier research
with potted trees indicated. The continuous growth and rapid turn-over of short-lived leaves in avocado results in a vegetative bias that favors potential shading of photosynthetically efficient source
leaves by younger leaves which are sinks for photoassimilates for about 40 days. However, except
for spring when shoot growth is synchronised by flowering, not all of the tree flushes at the same
time thus, much of the canopy remains well lit. Nevertheless, avocado management should focus
on tree shaping that allows a greater proportion of leaves to receive sufficient light to attain their
maximum photosynthetic potential. The considerable difference in photosynthetic responses to
temperature among the 3 avocado races and the fact that avocado trees can hybridize freely
among races (Whiley and Schaffer, 1994), creates a potential for breeding and/or selecting avocado cultivars for almost any environment within the temperature tolerance range for the species.
Also, photoinhibition of avocado leaves due to low temperatures can be quickly determined using
chlorophyll fluorescence techniques. Thus the potential exists to screen avocado selection for sensitivity to cold temperatures by using chlorophyll fluorescence on young plants or possibly even
detached leaves in temperature chambers.
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Avocado photosynthesis is sensitive to high evaporative demand and low soil moisture. However,
these responses are primarily a result of reductions in stomatal conductance. In fact, stomatal
conductance is an excellent early indicator of soil moisture stress in avocado and provides an
excellent “tool” for irrigation scheduling in avocado orchards (Neuhaus, 2003).
Avocado trees are sensitive to high salinity and the sensitivity varies among species. Although photosynthetic of avocado is reduced in response to salinity stress, there are not sufficient differences
among cultivars to use photosynthesis as an indicator of a cultivars ability to tolerate high salinity.
Enhancing the atmospheric CO2 concentration reduces the photosynthetic efficiency (amount of
carbon fixed per CO2 molecule) of mature avocado trees and in-vitro plantlets derived from tissue
culture. Despite the reduced photosynthetic efficiency elevating the atmospheric CO2 concentration increases the actual photosynthetic rate due to constant exposure to saturating CO2 concentrations. This has implications for global climate change, indicating the avocado may thrive under
elevated atmospheric CO2.
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